Electric resistor



Aug- 28, 1945. R. H. SCHMITT 2,383,823

ELECTRIC RESISTOR Filed June 15. 1943 Inventor: Richard H. Schmitt,

His Attorne.

Patented Aug. 28, 1945 ansc'rmo assrs'roa mslm-a n. schmitt, Schenectady, N. Y., signor to General Electric Company, a corporation of New York appusmonnnels, 194s, serial No. 490,812

4mm. (o1. zal-s1) My invention relates to electric resistors, particularly to resistors used in controlling electric circuits, and has for its object a simple and reliable resistor which is adapted to withstand severe shocks.`

In carrying out my invention, I provide a rigid supporting member about which the resistance conductor is wound, the supporting member being made of an electrically conducting material so as to serve also as an electric conducting lead or terminal for one end of the resistor. Between the turns of the resistor I apply a mass of hardened powdered electrically insulatingy material which, upon application, is mixed with water to form a plastic mass. Outside of this insulating material I provide a layer of corrugated mica and enclose the whole in a cylindrical'sheath.

For a more complete understanding of my invention, reference should be had to the accompanying drawing, Fig. 1 of which is a view in section of an electric resistor embodying 'my invention; Fig. 2 is a sectional view taken along the line 2-2 of Fig. 1 looking in the direction of the arrows but with the powdered insulating material removed so as to disclose the inner construction of the terminal arrangement; Fig. 3 is a fragy mentary view showing the insulator and terminais at one end of the resistor; while Fig. 4 is an enlarged fragmentary view of the sheet of corrugated mica insulating material. f

Referring to the drawing, in carrying out my invention in one form I provide a cylindrical supporting or terminal rod i0, which is also an electric conductor, so as to form one terminal connection for the resistor. Preferably this rod I0 is made of nickel because of the low resistance of nickel. as well as its ability to withstand high temperatures without detrimental oxidation. The rod is provided with a layer of electrically insulating material il, preferably a layer of vitreous enamel, which layer extends through the turns of the helical electric resistor i2 surrounding the rod I0. One end I3 of the resistor jl2sis electrically connected to the rod near'one end,

case the resistor is small and has a relatively high resistance, and the resistance conductor is not stiif enough to be self-supporting. It is therefore provided with the insulation covering It and then wound directly on the insulated terminal rod Ill.

At its right-hand end., as seen in Fig. 1 of the drawing, the terminal rod extends through a square terminal supporting block or insulator I1 made of a suitable electrically insulating material, such as porcelain. The block has .an inner smaller square portion It which extends loosely through a square hole is in the hat terminal member I5. Also, three lugs 20,'2I and 22 are l provided on the terminal member which extend a short distance alongthe square portion Il so as to further secure the iiat terminal Il in its transverse relation with respect to the portion I8. 'I'he opposite side, or right-hand side as seen in Fig. 1

1, of the fiat terminal is seated against the shoulder formed between the smaller portion I8 and the outer larger square portion of the terminal support i1, The rod iii extends through a central opening in the porcelain support, is bent at right angles downward, as seen inFig. 1, and lies in a slot 22a in the insulator. A cavity 22D is provided in the insulator for the bend in the terminal rod. This cavity is nlled with a suitable insulating material 22e. preferably a vitreous enamel which secures the two parts together.

A mass 23 of hardened electrically insulating material surrounds the resistor and the terminal rod. Outside of this mass of insulatingl material is a layer 24 of corrugated mica and the whole assembly is contained in an outer cylindrical metal sheath 25, preferably made of nickel, which is enlarged around the insulator 'projection I8. A porcelain insulator 26 surrounds the left-hand end of the terminall rod, as seen in F18. 1, and

-holds it centrally of the sheath. This insulator 'is secured in place by crimping or forming pe.- l ripheral indentations in the sheath at points 21 and 2l on each side of the insulator. The sheath is additionally crimped at the point 29 at its other end so as to secure it to the mass of insulatin material23 and to the terminal insulator` i1. A ring of suitable insulating material, preferably mica, is provided` at the end of the sheath between the sheath and the flat terminal I6.

In the assembly 'ofthe resistor the terminal rod l0 is preformed with the right angle terminal vextensionv and connection loop Il at one end, and then it is inserted into the insulator l1 through a hole provided for'it to the final position shown in Fig.y 1. 'I'hen the rod I0 is secured' to the insulator by applying the mass of vitreous -enamel 22e. In the next step the resistance oonductor is wound on the insulated portion of the terminal rod and the brazed connections are made for the ends of the resistance conductor with the rod and theflat terminal Il.

'I'hesheath 25 is prepared for assembly by inserting the insulator .2B in its smaller end and -securlng itin place by spinning and contracting the sheath on each side of it at the points Il and 28.l Also, the layer 2l of corrugated mics. is put inside of the sheath. This is done by forming the mica into a cylinder with the corrugations running lengthwise of the cylinder and inserting the cylinder in the sheath. A tool is then inserted to force the mica outward in place against the inner surface of the sheath. Preferably, the edges of the mica overlap somewhat at the longitudinal joint between them. Also, at this time the groove 28 is formed in the outer enlarged end of the sheath.

A predetermined quantity of insulating material 23 inthe form of a thick liquid or paste is used. Preferably this material is s fine powder consisting of a mixture of sodium silicate, feldspar, flint and waterglass. Water is added to the powder until a fairly thick liquid is formed, somewhat similar in plasticity to cake batter. A predetermined quantity of this thick liquid is then put into the sheath which is held upright with the insulator 28 lowermost. It will be. understood that the material is sufficiently solid so that no appreciable amount runs through the opening in the insulator ,26. As a matter of fact, in a typical device the dimensions of the sheath were about one-thirdV ofthe dimensions of the patent drawing, and the hole in the insulator 28 about one-eighth inch in diameter.

Before inserting the terminal rod and resistor assembly in the sheath. a small amount ofthe insulating material is applied to the resistor between the turns. This is done by inserting the terminal rod and the resistor assembly in a container containing a quantity of the prepared insulating material. the container being rotated as by a suitablemotor. In this step the terminal rod is held upright with the insulator uppermost and inserted in the insulating material over the lengthof the resistor, a'nd also the reduced portion I8 of the insulator. The resistor is then removed from the insulating material and, with the relatively small quantity adhering to it between the turns, is inserted in the sheath. To facilitate this assembly, the terminal rod is appreciably smaller than the hole in the insulator 2|, and also the wall of the hole is beveled at its inner end so as to guide the terminal into the hole.^ The resistor and insulating assembly is pressed into the sheath to its final position, a small excess of the insulating material being forced out at the enlarged right-hand end, as seen in Fig. l, of the sheath. Possibly some of the insulating material will be forced out through the insulator 26 at this time becausegof the relatively large clearance around the terminal rod.

Preferably. several holes 32 (Fig. 4) are provided in the layer of mica 2l vto provide for small quantities of the insulating material being forced through the holes 32 into direct engagement with the inner surface of the sheath. This provides for increased rigidity of the assembly and prevents loosening of the parts when the device is subjected to extreme shocks'. Also the insulating material when dry adheres firmly to the sheath at the points where the insulating material extends through the holes and thus secures the sheath against longitudinal movement off the terminal rod during severe shocks.

After the assembly of the unit as previously described, the insulating material!! is dried to a solid compact mass. Another advantage of the substantial clearance between the insulator 20 and the terminal rod is to provide for the escape of moisture at that point. Preferably, the drying operation is carried out slowly at first to assure that moisture from the inside portions escapes before the outer portions have hardened to form a seal. I preferably first .dry the unit for twelve h ours while holding it over a vessel containing water so as to provide a moist atmosphere. After this preliminary drying operation, the unit is placed in an oven and heated lto a temperature of about 2G0 C. for three hours. This thoroughly dries the insulating material and forms it into a compact hardened mass. During the oven drying operation, the moisture escapes rapidly through the clearance space provided between the terminal rod and the insulator 28. It will be understood that this clearance space will be filled with the insulating material, but the insulating material has sufficient porosity at this time to allow the escape of the remainder of the moisture.

For the purpose of providing for more efficient radiation of heat from the unit, the nickel sheath 2l preferably has its outer surface oxidized to give it a dark finish. This may be done by heating the sheath prior to assembly to a temperature of l,000 C. for fifteen minutes.

The sheath is further anchored in place on the hardened insulating material bythe grooves 2l and 29 at each end. Also, the insulation fills grooves (not shown) in the comers of the insulator portion i8, as weil as embedding the terminal connection member Il and other portions of the 4o insulator I8 thereby further to secure the insulatorinplace.

When resistance conductors or resistors of larger cross section, giving lower resistance, are

used, the resistor itself need not be covered with a layer of electrically insulating material. This particularly applies to resistor wires or strips having sufficient rigidity to be self-sustaining or selfsupporting between their ends so that the resistor can -be preformed into a helix whose turns are considerably larger than the diameter of the terminal rod lil and then slipped over the rod and the ends of the resistor securedI as previously described, so that the turns are spaced from the terminal rod as well as from each other. In such case. the insulating material is forced between the turns, and also betweenthe turns and the terminal rod, so as to thoroughly insulate the resistor.

The layer 24 of corrugated mica is of particular advantage when the resistor turns are considerably larger than the rod Il in assuring that the resistor is electrically insulated from the outer sheath Il. Thus when the plastic insulating material is being loaded into the sheath. the enlarged resistor turns might be displaced lat- Yerally sufficiently to engage the inner surface of a,sss,aas

to the ilat terminal'll, with the connector member Il extending parallel with the lug Il and perpendicular to the plane of the paper. This connector member extends along the side o! the square portion Il and projects beyond the leithand end of the square portion, as viewed in Fig; 1, where a suitable connection such as a brazed joint Il is provided between it and the end of the resistor I2.

While I have shown a. particular embodiment of my invention, it will be understood,.of course, that I do not wish to be limited thereto since many modications may be made, and I therefore contemplate by the appended claims to cover any such modincations as fall within the true Vspirit and scope of my invention. v

` lugs extending lengthwise oi said 'smaller portion thereby to secure said second terminal member inlaterally extending relation with said 'smaller portion of said insulator, an electric resistance conductor on said rod, electrical connections between the ends of said resistance conductor respectiveLv and said vlod andsaid second terminal, a mass of hardened insulating material on said rod surrounding said resistance conductor. a layer of corrugated mica surrounding said insulating material, said mica having its corrugations extending substantially lengthwise oi' said terminal rod, and an outer tubular metal sheath surrounding said mica and having an enlarged portion at one end surrounding said smaller por-l end terminal portion ot said rod, vitreous enamel insulating` material securing said rod and said insulator together, a small'rectangular portion on said insulator extending along said rod, a second terminal member provided with an aperture through which said rectangular portion extends and with integral lugs extending lengthwise of said rectangular portion thereby to secure said second terminal member in laterally extending relation with said rectangular portion oi said in-l sulator, an electric resistance conductor wound on said rod, electrical connections between the endsof said resistance conductor respectively and said rod and said second terminal, a mass' oi' hardened insulating material on said rod surrounding the turns yoi said resistanceconductor,

- and said rectangular portion o! saidy insulator. a

layer of corrugated mica surrounding said insulating material. saidV mica having its corrugations extending substantially lengthwise of said terminal rod, an outer tubular metal sheath surrounding said mica and having an enlarged portion `stone end sun'ounding said rectangular portion o! said insulator. an electric insulator between the end oisaid sheath and said second terminal member, and an insulator on the other end ot said rod for centering said rod in said sheath.

3. An electric resistor comprising an electric resistance conductor, an outer metallic sheath surrounding said `-resistance conductor inA spaced relation therewith, a layer oi corrugated mica inside said sheath in engagement therewith, said layer being provided with a plurality of apertures, and a mass oth'ardened electrically insulating material inside said layer surrounding said resistance conductor and extending through said apertures into engagement with said sheath.

4. An electric resistor comprising an electrically conducting terminal rod, an electric resistance conductor wound into turns surrounding said rod. metallic sheath surrounding said terminalrod and resistance conductor in spaced relation therewith, a layer of corrugated mica inside said sheath in engagement therewith, said layer of micabeing provided with' a plurality oi' apertures, and a mass oi' hardened electrically insulating material inside said layer of mica surrounding the turns of said resistance conductor and extending through said apertures into en-y gagement with said sheath.

- RICHARD H. BCHMI'I'I'. 

